1. Field of the Invention
The present invention relates to processes for making carbon-carbon composites, and more particularly to processes in which a composite comprised of carbon and a resin binder is treated to carbonize the resin binder and densify the composite to provide the composite with desirable properties including high temperature oxidation resistance.
2. History of the Prior Art
Carbon-carbon composites are useful in various applications including brakes for automotive vehicles and aircraft, rocket nozzles, and other missile components. Such composites are made from a mixture of carbon and a resin binder. The carbon is made from an organic material which has been carbonized or graphitized and which may assume various forms such as a woven fabric, fibers of selected length and diameter or a matte. In many instances such materials are mixed with a filler which is of carbon composition such as carbon black. The resin binder is an organic type which will char when subjected to high temperature processing so as to ultimately provide a carbon-carbon composite.
In prior art processes for making carbon-carbon composites the initial mixture of carbon material and an organic resin binder is treated such as by molding to provide the composite with a desired shape. The resin binder is at least partially cured by heating for a selected period of time. The resin binder is then carbonized such as by heating to 700.degree.-1000.degree. F. for 100-250 hours. The carbonization step is carried out very slowly so as not to delaminate or otherwise damage the composite. Delamination or other damage to the composite readily occurs due to the substantial decomposition of the resin and the large number of volatiles which are driven from the resin during carbonization. Following carbonization of the resin binder, the resin binder is typically pyrolyzed such as by heating the composite to 4000.degree.-4500.degree. F. for 12-40 hours.
At the end of the pyrolyzing step the composite is of substantially all-carbon composition but has less density than the original composite, is subject to oxidation at relatively low temperatures and otherwise exhibits poor physical properties. To increase the density and high temperature resistance of the composite, further processing steps are normally carried out. One approach is to reimpregnate and repyrolyze the composite. This may be done numerous times, and eventually results in a composite of relatively high density and therefore improved temperature resistance. Another approach is to subject the composite to chemical vapor deposition such as by placing the composite in a hot furnace in the presence of methane. This process when carried out for 100-400 hours provides the composite with some densification and high temperature resistance.
The multiplicity of different and separate steps requiring several days or longer in prior art processes for making carbon-carbon composites make such materials very expensive and impractical for many applications. In an effort to simplify the processing involved, certain techniques have been tried. An example of such techniques is provided by U.S. Pat. No. 3,734,797, Byers, issued May 22, 1973. The Byers patent recognizes that during carbonization of the resin binder in the formation of a carbon-carbon composite, large amounts of volatiles are released from the resin binder. The Byers patent goes on to disclose that by providing the composite with perforations and by applying pressure to the composite during carbonization, removal of the volatiles is accomplished without delamination or other damage to the composite. However the resulting composite is still far from having the requisite density and high temperature resistance for most applications, requiring that further processing steps of the type described above such as reimpregnation and repyrolyzation or chemical vapor deposition be employed. As a result the process for making the carbon-carbon composite, while somewhat simplified, is still far from being as simple and economical as it should be. Moreover, the end product still has lower density and high temperature oxidation resistance than is desired for such materials.
Accordingly it is an object of the present invention to provide a process for forming high density, high temperature resistant carbon-carbon composites.
It is a further object of the present invention to provide a process for making high density, high temperature resistant carbon-carbon composites which is not only simplified and economical but which provides for relatively reliable and uniform results.
It is a still further object of the present invention to provide a process for making high density, high temperature resistant carbon-carbon composites which is easily controlled and varied so as to optimize the process for certain applications and to optimize the properties of the resulting composite for particular requirements.
It is a still further object of the present invention to provide a process for making high density, high temperature resistant carbon-carbon composites in which the composites can assume a variety of different shapes and configurations while still taking advantage of a greatly simplified and more economical process.